Focal region optical elements for high-performance optical scanners

ABSTRACT

An optical device includes a light source, which is configured to emit a beam of light at a given wavelength, and at least one scanning mirror configured to scan the beam across a target scene. Light collection optics include a collection optic positioned to receive the light from the scene that is reflected from the at least one scanning mirror and to focus the collected light onto a focal plane, and a non-imaging optical element including a solid piece of a material that is transparent at the given wavelength, having a front surface positioned at the focal plane of the collection lens and a rear surface through which the guided light is emitted from the material in proximity to a sensor, so that the collected light is guided through the material and spread over the detection area of the sensor.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication 62/563,703, filed Sep. 27, 2017, which is incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates generally to opto-electronic devices, andparticularly to optical scanners.

BACKGROUND

Optical scanners are widely used for optically probing extended objectsor target scenes. In a typical scanner a light source sends out a beamof light; the beam is scanned across the object by, for instance, ascanning mirror; then the light returned from the object is collected bycollection optics and is directed to a sensor. The sensor emits a signalto a controller for further analysis.

High-performance optical scanners typically use both high-power lightbeams and high-speed scanners in order to sample and sense distanttarget scenes with high temporal and spatial resolution with a goodsignal-to-noise ratio.

SUMMARY

Embodiments of the present invention that are described hereinbelowprovide improved optical scanners and methods for scanning.

There is therefore provided, in accordance with an embodiment of theinvention, an optical device, including a light source, which isconfigured to emit a beam of light at a given wavelength, at least onescanning mirror configured to scan the beam across a target scene, and asensor having a detection area. Light collection optics include acollection optic positioned to receive the light from the scene that isreflected from the at least one scanning mirror and to focus thecollected light onto a focal plane. A non-imaging optical elementincludes a solid piece of a material that is transparent at the givenwavelength, having a front surface positioned at the focal plane of thecollection lens and a rear surface through which the guided light isemitted from the material in proximity to the sensor, so that thecollected light is guided through the material and spread over thedetection area of the sensor.

In a disclosed embodiment, the rear surface of the non-imaging opticalelement is in contact with the sensor, and the non-imaging opticalelement includes a compound parabolic concentrator (CPC). The materialmay include silicon or glass.

In some embodiments, the beam of light includes a beam of light pulses,and the sensor is configured to output a signal indicative of a time ofincidence of a single photon on the sensor. A controller is configuredto find times of flight of the light pulses to and from points in thescene responsively to the signal.

There is also provided, in accordance with an embodiment of theinvention, a method of sensing, which includes scanning a beam of lightat a given wavelength across a target scene using at least one scanningmirror. The light from the scene that is reflected from the at least onescanning mirror is collected and focused onto a focal plane. A frontsurface of a non-imaging optical element, including a solid piece of amaterial that is transparent at the given wavelength, is positioned atthe focal plane. The collected light is sensed using a sensor positionedin proximity to a rear surface of the non-imaging optical element, whichspreads the collected light over a detection area of the sensor.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of an optical scanner with a non-imagingoptical element, in accordance with an embodiment of the invention;

FIG. 2 is a schematic pictorial view of a non-imaging optical element,with optical rays traced through the element, in accordance with anembodiment of the invention; and

FIG. 3 is a schematic pictorial view of a non-imaging optical element,with optical rays traced through the element, in accordance with anotherembodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

High-performance optical scanners, for instance light detection andranging (LiDAR) sensors, typically use both high-intensity light beamsand high-speed scanners in order to sample and sense distant targetscenes with high spatial and temporal resolution with a goodsignal-to-noise ratio. The distance to the target scene may range fromtens of centimeters to hundreds of meters.

The beam emitted by the light source is scanned across the target sceneby a high-speed scanner, for instance by a rapidly oscillating scanningmirror. The light returned from the scene is reflected by a scanningmirror (either the scanning mirror described above or another mirrorsynchronized to the above mirror) towards a collection lens. An imagingcollection lens focuses the returned light into a small spot on thesensor, and the signal emitted by the sensor is received by a controllerfor further analysis.

High-speed scanners are employed in this application to maximize themeasurement rate. At very high angular speeds, the round-trip time forlight can introduce a lag angle at the receiver. In order to compensatefor deviations of the focused spot due to the scanner, a large sensingarea is required. Furthermore, small focused spots of both returnedlight and scattered light have a very high irradiance that can damagethe sensor and possibly degrade detection efficiency.

The lag angle γ is given by the expressionγ=2τω₀θ₀,wherein ω₀ and θ₀ are the scanner frequency and amplitude, respectively,and τ is the delay time, given byτ=2R/c,wherein R is the distance to the scene and c is the speed of light. Thelength L of the streak in the focal plane of the collection lens isgiven by the expressionL=2γf=4τω₀θ₀ f,wherein f is the focal length of the collection lens.

The embodiments of the present invention that are described hereinaddress the above limitations so as to enable high-speed,high-resolution optical scanners utilizing a sensor with high bandwidth,low dark noise, and reduced potential for damage and saturation. Thedisclosed embodiments use a non-imaging optical element, which isfabricated of a solid piece of dielectric material and is positionedbetween the focal plane of the collection optics and the sensor. Thisarrangement affords at least the following advantages:

-   -   1) The beam returned from the target scene and focused by the        collection optics is spread on the sensor over an area that is        larger than the spot in the focal plane;    -   2) The beam is stabilized on the sensor to a position that is        essentially independent of the mirror speed and target distance;        and    -   3) Using a solid piece of dielectric material for the        non-imaging optical element increases its acceptance angle        (numerical aperture).        The use of a non-imaging optical element enables reducing the        detection area of the sensor relative to what would otherwise be        required in the absence of the non-imaging optics, as well as        lowering the peak irradiance on the sensor. Both advantages        result in larger signal bandwidth and longer detection ranges        than could otherwise be achieved without the extra-focal        elements.

The disclosed embodiments of the present invention provide opticalscanners, wherein the light source can be either a non-laser source(such as a thermal source, a solid state source or a gas dischargesource) or a laser source (either continuous-wave or pulsed). Suchscanners can be used in depth-mapping systems, such as LIDARs, whereinthe light source is a pulsed laser source, the sensor is a single-photonavalanche diode (SPAD), and a controller finds the distance to thetarget scene by time-of-flight analysis.

In alternative embodiments, other high-sensitivity sensors, such as anavalanche photodiode (APD), may be used.

System Description

FIG. 1 is a schematic side view of an optical scanner 20 with anon-imaging optical element 21, in accordance with an embodiment of theinvention. The beam from a light source 22, such as, for example, acontinuous-wave or pulsed laser, emitted at a given wavelength, isdirected to a target scene 24 by a scanning mirror 26, forming andscanning an illumination spot 28 over the target scene. (The terms“light” and “illumination” are used herein to refer to any sort ofoptical radiation, including radiation in the visible, infrared, andultraviolet ranges.) The light returned from illumination spot 28 isreflected by scanning mirror 26 and a beamsplitter 30 towards acollection lens 32, which focuses the light onto its focal plane 34 asrays 35.

Non-imaging optical element 21 has its front surface 37 positioned at orin proximity to focal plane 34 and its rear surface 39 at or inproximity to a sensor 38, such as a photodiode, a SPAD, or an APD.Non-imaging optical element 21 is fabricated of a solid piece ofmaterial that is transparent at the given wavelength and is configuredto spread the light focused by collection lens 32 over the detectionarea of sensor 38. Front and rear surfaces 37 and 39, respectively, arecoated with a suitable anti-reflection coating in order to minimize thereflection losses at these surfaces.

A controller 40 is connected to light source 22, scanning mirror 26, andsensor 38. Controller 40 typically comprises a programmable processor,which is programmed in software and/or firmware to carry out thefunctions that are described herein. Additionally or alternatively, atleast some of the functions of controller 40 may be carried out byhardware logic circuits, which may be hard-wired or programmable. Ineither case, controller 40 has suitable interfaces for receiving dataand transmitting instructions to other elements of the system asdescribed. Thus, for example, controller 40 can be coupled to drivelight source 22 and scanning mirror 26, as well as to receive andanalyze signals emitted by sensor 38. In a LIDAR used for mapping thedepth of target scene 24, for example, light source 22 comprises apulsed laser, and sensor 38 comprises a SPAD. Controller 40 measurestimes of arrival of photons at sensor 38 relative to pulses emitted bylight source 22 in order to derive time-of-flights across the targetscene and thus to produce a depth map of the target scene.

As will be further detailed below, proper choice of non-imaging opticalelement 21 enables the use of a sensor 38 with a detection area havingsmaller dimensions than would be required if the sensor were locateddirectly in focal plane 34. The use of a smaller detection area yields ahigher bandwidth and a lower dark noise as compared to a larger sensor.Furthermore, non-imaging optical element 21 spreads the light focused atfocal plane 34 over the detection area of sensor 38, thus alleviatingproblems that are associated with a high local irradiance.

FIG. 2 is a schematic pictorial view of non-imaging optical element 21,fabricated of solid silicon, with optical rays 35 traced through theelement, in accordance with an embodiment of the invention. In thisembodiment, non-imaging optical element 21 comprises a compoundparabolic concentrator (CPC) and functions as a light-pipe to guidelight from front surface 37 to rear surface 39.

A CPC is an optical element comprising a rotationally symmetricalsurface around the optical axis, and planar front and rear surfacesperpendicular to the optical axis. The shape of the rotationallysymmetrical surface is defined by rotating a section of a parabola(parabolic section) around an axis. This axis is defined as theperpendicular bisector of the line connecting the focal point of theparabola to the point of the parabolic section closest to the focalpoint. The surface defined by the rotation of this connecting line formsthe rear surface of the CPC. The front surface of the CPC is determinedby a suitable choice of the end-point of the parabolic section away fromthe rear surface. The maximal acceptance angle of a hollow CPC, i.e., aCPC that is filled with air, is typically tens of degrees, and is equalto the tilt angle between the axis of the parabola and the axis ofrotation. The maximal acceptance angle for a solid CPC, i.e., a CPCfabricated of a solid piece of material, is increased from that of ahollow CPC with the same geometry due to the refraction of the opticalrays at the front surface according to Snell's law. Consequently, asolid CPC is able to gather a larger angular extent of light than ahollow CPC. A hollow CPC is manufactured typically of glass or metal,and its inside is coated with a suitable reflective material, such asaluminum. A solid CPC is manufactured of a material that is transmissiveat the wavelength of interest, and its rotationally symmetrical surfacemay be coated externally with a suitable reflective material, such asaluminum.

As shown in FIG. 1, non-imaging optical element 21 is positioned withfront surface 37 at or in proximity to focal plane 34, and with rearsurface 39 in contact with or in proximity to sensor 38.

In the present embodiment, non-imaging optical element 21 is fabricatedof silicon, and is used in scanner 20 at a wavelength of 1550 nm. Atthis wavelength, silicon is highly transmitting, with an index ofrefraction of 3.5. Front and rear surfaces 37 and 39, respectively, arecoated with a suitable anti-reflection coating in order to reducereflection losses and to increase the overall transmission ofnon-imaging optical element 21. A rotationally symmetrical surface 50 ofnon-imaging optical element 21 does not require a reflective coating dueto the high index of refraction of silicon, which ensures total internalreflection of rays 35.

Rays 35 impinge on front surface 37 at a point 52, chosen in thisexample as the center point of the front surface. Rays 35 propagatewithin non-imaging optical element 21, internally reflecting fromrotationally symmetrical surface 50, and exiting through rear surface39. At rear surface 39 rays 35 have spread out to a substantially largerarea than at point 52 on front surface 37, thus alleviating the problemsthat are associated with a high local irradiance, were sensor 38 to beplaced at focal plane 34. Rays impinging on other points on frontsurface 37 propagate similarly within non-imaging optical element 21 torear surface 39.

Thus, independently of the position of focused rays in focal plane 34,all the rays transmitted by non-imaging optical element 21 arrive withinthe detection area of sensor 38, thus enabling the use of a sensor whosedimensions are independent of target distance and scan speed as long asthe streak length L is less than the diameter of front surface 37. As anexample, applying the equations presented above, a lag angle γ of 0.75°and a focal length f of 13.6 mm give a streak length L of 356 μm. Thusfor any scan speed and target distance yielding a lag angle notexceeding 0.75°, the diameter of front surface 37 does not have toexceed 356 μm.

The condenser efficiency of non-imaging optical element 21, defined asthe ratio between the areas of front surface 37 and rear surface 39, is12.25. Consequently, sensor 38 can be substantially smaller than wouldbe required if the sensor were positioned at focal plane 34.

The numerical aperture of rays accepted by the non-imaging opticalelement is 0.76. The transmittance of non-imaging optical element 21 is98%.

FIG. 3 is a schematic pictorial view of a non-imaging optical element60, fabricated of solid glass, with optical rays 35 traced through theelement, in accordance with another embodiment of the invention. As inthe preceding embodiment, non-imaging optical element 60 comprises acompound parabolic concentrator (CPC) functioning as a light-pipe toguide light from a front surface 62 to a rear surface 64 of the element.With reference to FIG. 1, non-imaging optical element 60 is positionedwith front surface 62 at or in proximity to focal plane 34, and withrear surface 64 in contact with or in proximity to sensor 38.

Non-imaging optical element 60 is fabricated of solid glass. Similarlyto non-imaging optical element 21 fabricated of silicon, front and rearsurfaces 62 and 64 are coated with a suitable anti-reflection coating inorder to reduce reflection losses and to increase the overalltransmission of non-imaging optical element 60. The transmittance ofnon-imaging optical element 60 fabricated of glass is 90%. Arotationally symmetrical surface 66 of non-imaging optical element 60may be coated with a suitable reflective material, such as aluminum.

Rays 35 impinge on front surface 62 at a point 68, again chosen by wayof example as the center point of the front surface. Rays 35 propagatewithin non-imaging optical element 60, internally reflecting fromrotationally symmetrical surface 66, and exiting through rear surface64. At rear surface 64 rays 35 have spread out to a substantially largerarea than at point 68 on front surface 62, thus alleviating the problemsthat are associated with a high local irradiance. Rays impinging onother points on front surface 62 propagate similarly within non-imagingoptical element 60 to rear surface 64. Thus, independently of theposition of focused rays 35 in focal plane 34, all the rays transmittedby non-imaging optical element 60 arrive within the detection area ofsensor 38, enabling the use of a sensor whose dimensions are independentof target distance and scan speed as long as the streak length L is lessthan the diameter of front surface 62.

The condenser efficiency of non-imaging optical element 60 is 4, thenumerical aperture is 0.76, and non-imaging optical element 21 canaccept a lag angle γ of 0.4°.

Although non-imaging optical elements 21 and 60 both comprise a CPC,other embodiments of the present invention can use other sorts ofnon-imaging optical elements comprising a solid piece of a material thatare known in the art, such as various sorts of light-pipes.

It will thus be appreciated that the embodiments described above arecited by way of example, and that the present invention is not limitedto what has been particularly shown and described hereinabove. Rather,the scope of the present invention includes both combinations andsubcombinations of the various features described hereinabove, as wellas variations and modifications thereof which would occur to personsskilled in the art upon reading the foregoing description and which arenot disclosed in the prior art.

The invention claimed is:
 1. An optical device, comprising: a lightsource, which is configured to emit a beam of light at a givenwavelength; at least one scanning mirror configured to scan the beamacross a target scene; a sensor having a detection area; and lightcollection optics comprising: a collection optic positioned so thatafter the light emitted from the light source is reflected from thescene, and a part of the light that is reflected from the scene isreflected from the at least one scanning mirror, the collection opticcollects and focuses the light that was reflected from the at least onescanning mirror onto a focal plane; and a non-imaging optical elementcomprising a solid piece of silicon that is transparent at the givenwavelength, having a front surface positioned at the focal plane of thecollection optic and a rear surface through which the light that hasbeen focused by the collection optic and guided through the silicon isemitted from the silicon in proximity to the sensor, so that the lightis spread over the detection area of the sensor.
 2. The optical deviceaccording to claim 1, wherein the rear surface of the non-imagingoptical element is in contact with the sensor.
 3. The optical deviceaccording to claim 1, wherein the non-imaging optical element comprisesa compound parabolic concentrator (CPC).
 4. The optical device accordingto claim 1, wherein the beam of light comprises a beam of light pulses,and wherein the sensor is configured to output a signal indicative of atime of incidence of a single photon on the sensor.
 5. The opticaldevice according to claim 4, and comprising a controller, which isconfigured to find times of flight of the light pulses to and frompoints in the scene responsively to the signal.
 6. A method of sensing,comprising: scanning a beam of light at a given wavelength across atarget scene using at least one scanning mirror; collecting the lightthat was reflected from the scene and then reflected from the at leastone scanning mirror and focusing the collected light onto a focal plane;positioning a front surface of a non-imaging optical element, comprisinga solid piece of silicon that is transparent at the given wavelength, atthe focal plane; and sensing the collected light using a sensorpositioned in proximity to a rear surface of the non-imaging opticalelement, which spreads the collected light over a detection area of thesensor.
 7. The method according to claim 6, wherein the sensor ispositioned in contact with the rear surface of the non-imaging opticalelement.
 8. The method according to claim 6, wherein the non-imagingoptical element comprises a compound parabolic concentrator (CPC). 9.The method according to claim 6, wherein the beam of light comprises abeam of light pulses, and sensing the collected light comprises sensingthe light pulses and emitting a signal indicative of a time of incidenceof a single photon on the sensor.
 10. The method according to claim 9,and comprising finding times of flight of the light pulses to and frompoints in the target scene responsively to the signal.